Symbiotic X-ray binary

A symbiotic x-ray binary (SyXB) is a type of low-mass X-ray binary (LMXB) containing a neutron star or black hole accreting off of the stellar wind of a K- or M-type giant. SyXBs are characterized by their unique method of accretion, long neutron star spin periods, long orbital periods, and low luminosities. Although the first SyXB was discovered in the 1970s, very few have been discovered since, and no black hole SyXBs have been found at all. SyXBs are believed to be disproportionately the products of electron-capture supernovae.

Formation

Diagram showing the evolutionary path of X-ray binaries containing neutron stars. They start as main-sequence/main-sequence primordial binaries, then become supernovae with a neutron star and a main-sequence star, and then become a a detached binary with a neutron star and a main-sequence star. If the binary has a close orbit, it will become a low-mass X-ray binary and then eventually a millisecond pulsar/white dwarf binary. If the binary has a wide orbit, it will become a symbiotic X-ray binary and then eventually a neutron star/white dwarf binary.
Evolutionary tracks of neutron star/main-sequence binaries. If the pair has a wide orbit, it will evolve into an SyXB.

When a neutron star forms in a supernova, it generally experiences a supernova "kick" that changes its velocity. In the case of SyXBs, the neutron star must experience a relatively small kick—less than 1/6 of typical—in order to stay bound to its companion. Because of this, it has been suggested that SyXBs may be disproportionately the results of electron-capture supernovae.

Properties

SyXBs accrete from the donor star's stellar wind and, unlike most other LMXBs, do not experience Roche lobe overflow. They are analogues to the cataclysmic variable symbiotic stars, where the accretor is a white dwarf instead of a neutron star. They also display similar properties to Be/X-ray binaries, which also contain a compact object accreting off of the stellar wind of another star, although in the case of Be/X-ray binaries, the donor star is high-mass.

SyXBs have relatively low X-ray luminosities of 1032–1036 erg s-1. Their X-ray spectra are very hard and variable, and their optical spectra lack emission lines and are dominated almost completely by the donor star. SyXBs have orbital periods of months to years, some of the highest orbital periods of any LMXBs. They also have very slowly-rotating compact objects compared to other known LMXBs. Nearly all SyXBs are transients, with only one persistent SyXB known (GX 1+4).

Observation

Two light curves showing an oscillatory, sine-wave-like pattern
Light curves of SyXB IGR J16194–2810

The first SyXB was discovered in 1977; despite their long observational history, however, SyXBs remain relatively rare. There are expected to be hundreds to thousands of SyXBs in the galaxy, the vast majority of which have not yet been found.

As of December 2024, no black hole SyXBs have been found. However, this does not necessarily mean they cannot exist. Black hole SyXBs would likely have a very low X-ray luminosity because much of the accreted material would either fall into the black hole or be lost as outflows.

As of July 2024, 3 black holes and 21 neutron stars in AU-scale orbits with low-mass stars have been found by the Gaia telescope. Although they are currently quiescent, these binaries are expected to eventually evolve into SyXBs.

Properties of known SyXBs

Name Rotational period (s) Orbital period (d) X-ray luminosity (erg s-1) Donor spectral type References
GX 1+4 120 1161 1035–1036 M5
4U 1954+31 18300 ≥400 4×1032–1035 M4
4U 1700+24 404 2×1032–1034 M2
Scutum X-1 113 2×1034 Late K/early M
IGR J16194–2810 14616 192.73 ≤7×1034 M2
IRXS J180431.1–273932 494 ≤6×1034 M6
IGR J16358–4724 5850 3×1032–3×1036
IGR J16393–4643 912 50.2
2XMM J174016.0–290337 626 ~3×1034 K1
CGCS 5926 151 ≤3×1032 Carbon star
CXOGBS J173620.2–293338 3.0×1031–7.2×1032 Carbon star
XTE J1743–363 1.1×1036 M8
CXOGBS J174614.3−321949 ~8.4×1032 (low-luminosity state)
~1.7×1034 (high-luminosity state)		 Early M
3XMM J181923.7–170616 407.9 K3
XMMU J174445.5–29504 ≥4×1034
IGR J17329–2731 6680 3×1035
IGR J17197–3010 ≤1.6×1035
CXOGBS J174614.3−321949 1034
CXOGBS J173620.2−293338 ≤1033
Swift J2037.2+4151 1036

References

  1. 1 2 3 4 5 6 7 Yungelson, Lev R.; Kuranov, Alexandre G.; Postnov, Konstantin A. (2019). "Wind-accreting symbiotic X-ray binaries". Monthly Notices of the Royal Astronomical Society. 485: 851–860. doi:10.1093/mnras/stz467.
  2. 1 2 3 4 5 Nagarajan, Pranav; El-Badry, Kareem; Lam, Casey; Reggiani, Henrique (2024). "The Symbiotic X-Ray Binary IGR J16194-2810: A Window on the Future Evolution of Wide Neutron Star Binaries from Gaia". Publications of the Astronomical Society of the Pacific. 136 (7). doi:10.1088/1538-3873/ad5dfd.
  3. 1 2 3 4 5 Wetuski, Joshua; Hynes, R. I.; MacCarone, T. J.; Heinke, C.; Torres, M A P.; Jonker, P. G.; Britt, Christopher T.; Steeghs, D.; Nelemans, G. (2021). "X-ray observations of two candidate symbiotic binaries in the galactic bulge". Monthly Notices of the Royal Astronomical Society. 506 (4): 5619–5628. doi:10.1093/mnras/stab2139.
  4. 1 2 3 4 5 Deng, Zhu-Ling; Li, Xiang-Dong (2024). "Are There Black Hole Symbiotic X-Ray Binaries?". The Astrophysical Journal. 977: 95. doi:10.3847/1538-4357/ad90ab.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 Kuranov, A. G.; Postnov, K. A. (2015). "Symbiotic X-ray binaries systems in the galaxy". Astronomy Letters. 41 (3–4): 114–127. doi:10.1134/S1063773715040064.
  6. 1 2 Lü, G.-L.; Zhu, C.-H.; Postnov, K. A.; Yungelson, L. R.; Kuranov, A. G.; Wang, N. (2012). "Population synthesis for symbiotic X-ray binaries". Monthly Notices of the Royal Astronomical Society. 424 (3): 2265–2275. doi:10.1111/j.1365-2966.2012.21395.x.
  7. 1 2 3 4 5 6 Bahramian, Arash; Degenaar, Nathalie (2024). "Low-Mass X-ray Binaries". Handbook of X-ray and Gamma-ray Astrophysics. pp. 3657–3718. arXiv:2206.10053. doi:10.1007/978-981-19-6960-7_94. ISBN 978-981-19-6959-1.
  8. 1 2 3 Qiu, Hao; Zhou, Ping; Yu, Wenfei; Li, Xiangdong; Xu, Xiaojie (2017). "3XMM J181923.7–170616: An X-Ray Binary with a 408 s Pulsar". The Astrophysical Journal. 847: 44. doi:10.3847/1538-4357/aa8728.
  9. ↑ Sidoli, Lara; Paizis, Adamantia (2018). "Investigating High Mass X-ray Binaries at hard X-rays with INTEGRAL". Proceedings of the International Astronomical Union. 14: 178–186. doi:10.1017/S1743921319001145.